Nested feed-forward optical equalization using an electro-optic modulator with a multi-segment electrode

ABSTRACT

A method and system of optical communication are provided. An optical modulator device includes a first and a second waveguide segment, and is configured to modulate an incident optical signal. A first feed-forward equalization (FFE) circuit including an inner first tap and an inner second tap, is configured to equalize the first waveguide segment. A second FFE circuit including a first inner tap and a second inner tap, is configured to equalize the second waveguide segment. An FFE recombination of the first inner tap and the second inner tap of the first and second FFE circuits, is in the electrical domain, respectively. An FFE recombination of the first and second modulation signals, operative to equalize a combination of the first second waveguide segments, is in the optical domain.

BACKGROUND Technical Field

The present application generally relates to telecommunication systems,and more particularly, to optical telecommunication systems utilizingequalization techniques.

Description of the Related Art

An electro-optic modulator is an optoelectronic device where asignal-controlled element having an electro-optic effect is used tomodulate a beam of light. The modulation may be with respect to thephase, frequency, amplitude, or polarization of the beam of light.Electro-optic modulators are largely used in fiber optical communicationsystems for realizing high-speed amplitude and phase modulation ofoptical signals. Various technologies and material platforms can be usedto realize an electro-optic modulator. The structure generally includesan optical waveguide that has a refractive index or an absorption thatis modulated by applying an RF electrical field that spatially overlapswith an optical mode of the waveguide.

After propagating through an L-long phase modulator, an input opticalsignal with a wavelength λ typically experiences a phase variationprovided by equation 1 below:Δφ=(2π/λ)ΔnL  (Eq. 1)

where,

-   -   Δφ is the phase variation;    -   Δn is the refractive index change due to the applied electric        field; and    -   L is the length of the modulator;

Electro-optic modulators can be used in various applications. Forexample, they can be used as straight-line elements to modulate theamplitude or the phase of an optical signal. Electro-optic phasemodulators can also be embedded in Mach-Zehnder structures or resonantstructures to realize advanced-modulation transmitters or opticalswitches. A Mach-Zehnder interferometer (MZI) is a structure that splitsa light beam in two optical signals that are phase-modulated relative toeach other and then recombined.

In order to improve gigabit transmissions in such communication systems,equalization techniques such as feedforward equalization (FFE) may beimplemented to, among other things, improve bit error rate (BER)performance in communication links.

SUMMARY

In one embodiment, an optical communication system, includes an opticalmodulator device having a first and a second waveguide segment, andconfigured to modulate an incident optical signal. A first feed-forwardequalization (FFE) circuit includes an inner first tap and an innersecond tap, and is configured to equalize the first waveguide segment ofthe optical modulator. A second FFE circuit including a first inner tapand a second inner tap, is configured to equalize the second waveguidesegment of the optical modulator. An FFE recombination of the firstinner tap and the second inner tap of the first FFE circuit that createsa first modulation signal to equalize the first waveguide segment, is inthe electrical domain. An FFE recombination of the first inner tap andthe second inner tap of the second FFE circuit that creates a secondmodulation signal to equalize the second waveguide segment, is in theelectrical domain. An FFE recombination of the first and secondmodulation signals, operative to equalize a combination of the firstsecond waveguide segments, is in the optical domain.

In one embodiment, a method of optical equalization using two layers offeed-forward equalization (FFE) on an optical modulator having a firstand a second waveguide segment is provided. In the first FFE layer, foreach waveguide segment, an electrical modulation signal is split into afirst inner tap signal and a second inner tap signal. The second innertap signal is delayed relative to the first inner tap signal. Anamplitude of the second inner tap signal is attenuated relative to thefirst inner tap signal. The delayed and attenuated second inner tapsignal is recombined with the first inner tap signal in an electricaldomain to create an FFE modulation signal. The FFE modulation signal isapplied to the corresponding waveguide segment. In the second FFE layer,a combination of the first and second waveguide segments is equalizingby recombining each FFE modulation signal in an optical domain.

These and other features will become apparent from the followingdetailed description of illustrative embodiments thereof, which is to beread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are of illustrative embodiments. They do not illustrate allembodiments. Other embodiments may be used in addition or instead.Details that may be apparent or unnecessary may be omitted to save spaceor for more effective illustration. Some embodiments may be practicedwith additional components or steps and/or without all of the componentsor steps that are illustrated. When the same numeral appears indifferent drawings, it refers to the same or like components or steps.

FIG. 1A illustrates an example feed-forward equalization circuit.

FIG. 1B illustrates example waveforms related to the feed-forwardequalization circuit of FIG. 1A.

FIG. 2 illustrates a feed-forward optical equalization apparatus usingan optical modulator, consistent with an exemplary embodiment.

FIG. 3 illustrates an example Mach Zehnder Modulator device that is usedin a push-pull configuration.

FIG. 4 illustrates a nested feed-forward optical equalization apparatus,consistent with an exemplary embodiment.

FIG. 5 illustrates a perspective view of a cross-section of anelectrode, consistent with an exemplary embodiment.

FIG. 6A illustrates a generalized block diagram of a feed-forwardequalizer, consistent with an exemplary embodiment.

FIG. 6B illustrates a block diagram of a feed-forward equalizationimplementation in an electronic IC, consistent with an exemplaryembodiment.

FIG. 6C illustrates another block diagram of a feed-forward equalizationimplementation in an electronic IC, consistent with an exemplaryembodiment.

FIG. 7 illustrates a schematic of an assembled transmitter sub-system,consistent with an exemplary embodiment.

FIG. 8 illustrates an example experimental setup for a feed forwardequalization test configuration.

FIG. 9A illustrates eye diagrams taken at different transmitter outputspeeds.

FIG. 9B illustrates bit error rate plots of sensitivity for differentfeed forward equalization test configurations.

FIG. 9C illustrates bit error rate plots of timing margin for differentfeed forward equalization test configurations.

DESCRIPTION

In the following detailed description, numerous specific details are setforth by way of examples in order to provide a thorough understanding ofthe relevant teachings. However, it should be apparent to those skilledin the art that the present teachings may be practiced without suchdetails. In other instances, well known methods, procedures, components,and/or circuitry have been described at a relatively high-level, withoutdetail, in order to avoid unnecessarily obscuring aspects of the presentteachings.

The one or more exemplary embodiments described herein provide, amongother things, a FFE scheme that is generated within the optical domainusing optical modulator devices such as, without limitation, a MachZehnder Modulator (MZM), an Electro-Absorption Modulator (EAM), etc.

Referring to FIG. 1A, an exemplary FFE circuit implementation isdepicted. The FFE circuit 100 may include one or more pre-amplifierdevices 102, a first electrical path P1 (i.e., sometimes referred toherein as path Tap 0) having a buffer device 104, and a secondelectrical path P2 (i.e., sometimes referred to herein as path Tap 1)having one or more variable propagation delay buffer devices 106 and aninverter device 108.

The pre-amp 102 may receive, a modulation signal at its input 110. Themodulation signal at the input 110 may be a pulse amplitude modulation(PAM) signal, which is a form of signal modulation where the content isencoded in the amplitude of a series of signal pulses. The amplitudes ofthese series of pulses are varied according to the sample value of themessage signal. In various examples, the amplified pulse amplitudemodulation signal may be PAM-2, PAM-4, PAM-8, etc., where the PAM numberindicates the distinct number of pulse amplitudes that are used toconvey the information.

At the output 112 of the one or more pre-amplifier devices 102 theamplified modulation signal is split along paths Tap 0 and Tap 1. Inparticular, the output 112 of the pre-amplifier devices 102 that issplit along path Tap 0, as indicated by P1, is applied to buffer 104.The output 112 of the one or more pre-amplifier devices 102 that issplit along path Tap 1, as indicated by P2, is however, also applied tothe one or more variable delay buffer devices 106 and then to theinverter device 108. Accordingly, the same signal is applied to theinput of the main (i.e., Tap 0) buffer 104 as the signal that is appliedto the input of the one or more delay buffers 106. In variousembodiments, the split may be passive (e.g., a node) or active (e.g.,via a fan-out of the pre-amp 112. The FFE circuit output is locatedwhere paths Tap 0 and Tap 1 recombine, as indicated by node 114. In theexample of FIG. 1A, the recombination is performed in the electricaldomain. For example, in one embodiment, current mode logic is used tosum a current at the output of the first path P2 (i.e., at the output ofthe buffer 108) with the current of the first path (i.e., at the outputof the main buffer 104, at node 114). Alternatively, other knownsummation circuits may be used to perform the summation of the signalsof the first path P1 and the second path P2 at node 114.

It is emphasized that in the example of FIG. 1A, the summation of bothpaths (i.e., Tap 0 and Tap 1), sometimes referred to herein as therecombination of the Taps, is performed in the electrical domain (i.e.,not in the optical domain). The resulting signal, sometimes referred toherein as the feed-forward equalized waveform, is applied to an opticaldevice (e.g., laser) that is configured to convert the FFE waveform intoan optical signal for the optical medium 118 (e.g., optical fiber).

The buffer device 104 in path Tap 0 may be a unity gain buffer,amplifier, or another buffer device capable of providing gain. WithinTap 1, the one or more variable delay buffer devices 106 may includecommon mode logic (CIVIL) buffer devices capable of having a tunablepropagation delay by controlling their tail currents. Alternative, RFphase shifters may be utilized in place of the one or more variabledelay buffer devices 106. The inverter device 108 may include a CMOSinverter logic device. In various embodiments, different one or morepre-amplifiers 110, delay buffers 106, main buffer 104 and buffer 108may be used. It is believed that such components are generally known inthe art, and they are therefore not discussed here in detail forbrevity.

Referring now to FIG. 1B, the operation of circuit 100 is described withthe aid of waveform diagrams 115. Along path Tap 0, as indicated by pathP1, the output of the buffer 104 may be represented by waveform 120.Along path Tap 1, as indicated by P2, the output generated by the one ormore variable delay buffer devices 106 and the inverter device 108 isgiven by waveform 125. More specifically, the one or more variable delaybuffer devices 106 cause a predetermined delay (D_(L)) between theoutput of the buffer 104 represented by waveform 120 and the output fromTap 1 given by waveform 125.

Additionally, the delayed output from the one or more variable delaybuffer devices 106 is inverted by inverter device 108. In essence, thewaveform 125 generated from the output of Tap 1 is a delayed andcomplementary (i.e., inverted) version of the waveform 120 generated atthe output of Tap 0 (i.e., the output of the main buffer 104). Theamplitude of the output waveform 125 of Tap 1 is also attenuatedrelative to the output waveform 120 of Tap 0 to produce a tap weight(T_(w)). For example, a tap weight is an amplitude assigned to a givenTap (or branch) of the FFE circuit, relative to the other Taps of theFFE circuit. Waveform 120 from Tap 0 and waveform 125 from Tap 1 arethen combined to generate an FFE output waveform 130. As such, theamount of attenuation associated with the tap weight T_(w) along withthe predetermined delay D_(L), are utilized to generate an FFE outputwaveform 130 at the FFE circuit output located at where the Tap 0 andTap 1 paths recombine, as indicated at 114. By applying the FFE outputwaveform 130 to an optical device 116 (e.g., a laser), an equalizedoptical output waveform 135 having a desired impulse response isgenerated. In contrast, without the FFE output waveform 130, thebandwidth limited optical device 116 generally distorts a clean waveformthat may be provided to the optical device 116. For example, thewaveform exiting the optical waveguide 118 may exhibit inter-symbolinterference and may not provide reliable data transmission.Accordingly, the FFE waveform 130 that is combined in the electricaldomain and provided to the optical device 116 yields a more responsiveand reliable optoelectronic device by providing more sharp rise and falltimes.

However, there is a fundamental tradeoff between efficiency and speedwhen the optical device 116 is driven as a lumped element. For example,a longer optical device has inherently higher capacitance, therebyreducing performance as the larger the optical device 116 is. Putdifferently, to increase performance, the optical device 116 isgenerally made smaller when driven as a lumped element.

Reference now is made to FIG. 2, which illustrates a feed-forwardoptical equalization (FFOE) apparatus using an optical modulator 200,consistent with an exemplary embodiment. The FFOE apparatus using theoptical modulator 200 may include a multi-tap feed-forward equalization(FFE) circuit 204 that is coupled to an optical modulator device 202.The optical modulator device 200 may be any optical device that is asignal-controlled element that uses an electro optic effect or a thermooptic effect to modulate a beam of light that is provided at its input201. In various embodiments, the modulation may be imposed on the phase,frequency, amplitude, and/or polarization of the beam.

For discussion purposes, a multi electrode 206 Mach-Zehnder Modulator(MZM) is used as an optical modulator device 202 by way of example only,not limitation. The MZM 202 includes an input 201 configured to receivean input signal Osig, which is split into a first arm 205 and a secondarm 207. According to one example implementation (see FIG. 5), eachelectrode may include a segment (i.e., a length) of silicon waveguidewithin the modulation arm (i.e., Arm_2) of the MZM 202. The termselectrode and waveguide are used herein interchangeably. Based on theapplication of an electrical modulation signal across the segment ofsilicon waveguide, carriers (e.g., electrons) may be injected into orremoved from the silicon segment. By injecting or removing electricalcarriers (e.g., electrons), the refractive index, and thus, the opticalpath of the silicon segment is modulated or varied. As such, each pathP_(a)-P_(e) generates a refractive index change in each correspondingsilicon segment of the multiple electrodes or segments 206.

The FFE circuit 204 may include one or more pre-amplifier devices 210; afirst electrical path (i.e., Tap 0 path P_(e)) having a tunablepropagation delay buffer device 212, a tunable tap weight buffer device214, and a driver device 216; and a plurality of second electrical paths(i.e., Tap 1 paths P_(a) to P_(d)), each having one or more tunablepropagation delay buffer devices 220 a to 220 d. The one or morepre-amplifiers 210 may be shared between all paths (i.e., Pa to Pe) ofthe FFE circuit 204. In one embodiment, each path (i.e., Pa to Pe) hasits own pre-amplifier 210.

In some embodiments, one or more paths of Tap 1 (i.e., P_(a) to P_(d))include inverter devices, represented by inverters 222 a to 222 d.Accordingly, whether each of the inverters 222 a-222 d is included isbased on the desired control of a corresponding segment of the electrode206. Each path of Tap 1 (i.e., P_(a) to P_(d)) has an output driver 224a-224 d, respectively. As depicted, a modulation signal (e.g., a PAMsignal) may be applied to the input 208 of the one or more pre-amplifierdevices 210. The PAM signal (e.g., PAM-2, PAM-4, PAM-8, etc.,) at theoutput 211 of the one or more pre-amplifier devices 210 is then splitalong the Tap 0 path (P_(e)) and the Tap 1 paths (P_(a) to P_(d)). Thissplit of the modulated signal 208 that was amplified by thepre-amplifiers, is performed in the electrical domain.

In operation, the first electrical path (i.e., Tap 0 path P_(e)) maygenerate a signal from the split amplified PAM signal that is similar towaveform 120 of FIG. 1B. The second electrical paths (i.e., Tap 1 pathsP_(a) to P_(d)) generate a signal from the split PAM signals that issimilar to waveform 125 of FIG. 1B. In particular, in the example ofFIG. 2, the waveforms generated at the output nodes (O_(a) to O_(d)) ofthe second electrical paths (i.e., Tap 1 paths P_(a) to P_(d)) are timedelayed, and amplitude attenuated versions of the waveform generated atthe output (O_(e)) of first electrical path (i.e., Tap 0 path P_(e)). Insome embodiments, the waveforms are also inverted. The delays associatedwith the second electrical paths (i.e., Tap 1 paths P_(a) to P_(d)) maybe adjusted using the tunable propagation delay buffer devices 220 a to220 d so that each of the Tap 1 paths (P_(a) to P_(d)) interacts withthe same photons, since each propagating photon may be skewed by theoptical delay between the segments 206 of the MZM device 202. The sum ofthe waveforms generated at the outputs (O_(a) to O_(d)) of the secondelectrical paths (i.e., Tap 1 paths P_(a) to P_(d)) produce the overalltap weight associated with the Tap 1 paths P_(a) to P_(d). In addition,the tunable delay buffer device 212, along the first electrical path(i.e., Tap 0 path P_(e)), may provide an additional positive or negativedelay.

Variation of the delay and amplitude relations between the Tap 0 and Tap1 waveforms generate an equalization signal similar to the FFE waveform130 as illustrated in FIG. 1B, which is encoded onto the phase of theincident optical signal O_(sig) passing through the MZM device 202.

By virtue of recombining the paths of the first Tap (i.e., Tap 0, havingoutput Oe) and the second Tap (i.e., Tap 1, having output O_(a) toO_(d)) in the optical domain (i.e., not the electrical domain), over thesegmented electrode 206, many limitations of an optical device that isbeing driven as a lumped element are mitigated. For example, splittingthe Tap 1 paths over the multiple segments or electrodes 206 reduces thecapacitance seen by each driver (e.g., 216, and 224 a to 224 d) that isdriving a signal and modulating the phase by changing the effective pathlength of the segments. By segmenting the electrode 206, the effectivecapacitance of each segment becomes a fraction of the total length ofthe electrode, thereby improving the signal integrity and the bandwidthof the MZM device 202. Thus, the modulation bandwidth of the MZM device202 may be enhanced and the impulse response of the modulated opticalsignal more efficiently controlled by using the FFE process thatperforms the recombination of the Tap paths in the optical domain, asdiscussed herein.

Reference now is made to FIG. 3, which illustrates a FFOE apparatus in apush-pull configuration. The concepts and components of the FFOEapparatus of FIG. 3 are similar to those of FIG. 2, and many componentsare therefore not repeated here for brevity. For example, the push-pulloptical modulator 300 may include one or more pre-amplifier devices, atunable delay buffer device, a tunable tap weight buffer device, driverdevices, etc., similar to those of FIG. 2, except that the relevantpaths are now complementary, as illustrated in part in FIG. 3.Accordingly, the drivers 304 a to 304 e each receive a complementarysignal from their corresponding buffer (not shown). The output drivers304 a to 304 e each provide complementary outputs O_(1a) and O_(2a) toO_(1e) and O_(2e), respectively. The output of the first Tap may beprovided by the complementary outputs O_(1a) and O_(2a) of driver 304 a.The output of the second Tap may be provided by the complementaryoutputs (i.e., O_(1b) and O_(2b) to O_(1e) and O_(2e)) of drivers 304 bto 304 e, respectively, as illustrated in FIG. 3. Accordingly, both thefirst arm 305 and the second arm 307 of the MZM are controlled in acomplementary way by the push-pull FFOE apparatus 300. Similar to theexample embodiment of FIG. 2, the signals of both taps are recombined inthe optical domain.

Using such push-pull configuration, the length of segments 306 may beless than those of the single ended MZM device 202 of FIG. 2 forgenerating the same phase shift. Accordingly, referring back to equation1, for the same phase shift (Δφ), the length of the segment (L) can bemade shorter for the same optical wavelength λ of the incident opticalsignal. That is because the push-pull configuration of FIG. 3 enables todouble the relative change of the refractive index (Δn) between theMach-Zehnder arms generated by the applied electrical signal from theFFOE apparatus 300. For a given amount of voltage swing provided by eachdriver 304 a to 304 e, there is more phase modulation in the MZM 302.

Reference now is made to FIG. 4, which illustrates a nested FFOEapparatus 400. FIG. 4 is nested in that the feed-forward equalization isperformed in two layers. The first FFE is performed for each segment ofthe electrode (i.e., illustrated by way of example in FIG. 4 to have 5segments e₁ to e₅.). The first equalization is performed for eachsegment independently, in a way similar to the equalization discussed inthe context of FIGS. 1a and 1b . The second equalization is performedover all segments e₁ to e₅, in a way similar to the equalizationdiscussed in the context of FIG. 2. This nested equalization concept isdiscussed in more detail below.

The FFOE apparatus 400 of FIG. 4 illustrates that each segment e₁ to e₅of the electrode 442 is controlled by a corresponding FFE circuit 405 ato 405 e. A more detailed view of an FFE circuit 405 a is provided byway of a zoom view (represented by the dashed rectangle. Each of theremaining FFE circuits 405 b to 405 e may have a similar configurationto that of FFE circuit 405 a. Each FFE circuit (e.g., 405 a) may includeone or more pre-amplifier devices 402, a first electrical path P1 (i.e.,sometimes referred to herein as path Tap 0 A) having a buffer device404, and a second electrical path P2 (i.e., sometimes referred to hereinas path Tap 1 A) having one or more variable propagation delay bufferdevices 406 and an inverter device 408. In some embodiments, each FFEcircuit (e.g., 405 a) does not have a separate one or more pre-amplifierdevices 402; rather, a single set of one or more pre-amplifier devices402 is shared among all FFE circuits 405 a to 405 e.

The pre-amp 402 of each FFE circuit (e.g., 405 a) may receive, amodulation signal at its input 410. At the output 412 of the one or morepre-amplifier devices 402, the amplified modulation signal is splitalong paths Tap 0 A and Tap 1 A. In particular, the output 412 of thepre-amplifier devices 402 that is split along path Tap 0, as indicatedby P1, is applied to buffer 404. The output 412 of the one or morepre-amplifier devices 402 that is split along path Tap 1 A, as indicatedby P2, is however, also applied to the one or more variable delay bufferdevices 406 and then the inverter device 408. The recombination of bothpaths (i.e., Tap 0 A and Tap 1 A) of each FFE circuit (e.g., 405 a to405 e) of the first layer, is performed in the electrical domain (i.e.,not in the optical domain). The feed-forward equalized waveform, isapplied to a corresponding segment of the electrode 442 (e.g., segmente₁ for FFE circuit 405 a).

The second feed-forward equalization (e.g., second equalization layer)is performed over all segments e₁ to e₅. For example, the first tap(i.e., Tap 0 B) may include a single path, having an output O_(e) thatis coupled to segment e₅ of the electrode 442. The second tap (i.e., Tap1 B) may include one or more paths, represented by way of non-limitingexample as having outputs O_(a) to O_(d). Each of the outputs O_(a) toO_(d) is coupled to its segment e₁ to e₄ of the electrode 442,respectively. Significantly, the recombination of the secondfeed-forward equalization layer is performed in the optical domain. Byvirtue of having a nested feed-forward equalization having a first andsecond equalization layers, each segment of the electrode 442 is bettercontrolled, thereby providing a more efficient equalization across theelectrode 442.

Example Electrode Segment

With the foregoing overview of the different feed-forward equalizationcircuits, it may be instructive to discuss a physical implementation ofa single segment or electrode connected to a silicon optical waveguide.To that end, FIG. 5 illustrates a perspective view of a cross-section ofan optical waveguide, consistent with an exemplary embodiment. Tofacilitate this discussion, a symbolic representation of the one or moredelay buffer devices 220 a, inverters 222 a, and output driver 224 a ofFIG. 2 are superimposed on the segment of the electrode 500 to providebetter orientation. The electrode 512 may be viewed as part of a PNjunction of a diode 502 that is formed between the electrode 512 and avirtual ground 530 at the other side of the optical waveguide. Forexample, the electrode 512 might be connected to the N doped regionwhile the virtual ground 530 may be connected to the P doped region ofPN junction.

During forward biasing, carriers are injected into the Si waveguideregion creating a refractive index change, and thus, a phase change.Similarly, during reverse biasing, carriers are pulled away from the Siwaveguide region, creating a refractive index change, thereby creating acorresponding phase change.

Example Experimental Architecture

With the foregoing theoretical discussion of different feed-forwardequalization circuits and physical characteristics of a segment of anelectrode, it may be helpful to go over some actual experimentalresults. To that end, FIG. 6A illustrates a generalized block diagram ofa feed-forward equalizer, consistent with an exemplary embodiment. Theblock diagram of FIG. 6A illustrates that there is an input stage 602, asplit stage 604, a delay stage 606 a weight stage 608 a recombinationstage 610, and an output stage 612.

FIG. 6B illustrates a block diagram of an FFE implementation in anelectronic IC, consistent with an exemplary embodiment. FIG. 6A includesa pre-amplifier (PA) 620 and one or more variable delay amplifiers (VDA)622. In various embodiments, there may be one or more inverting ornon-inverting variable gain amplifiers (VGA) 624 communicatively coupledto the output of the VDA 622. There may be a recombining stage 626 thatmay be used to drive a single device 628 within an MZM. In FIG. 6A, therecombination stage is in the electrical domain.

FIG. 6C illustrates another block diagram of an FFE implementation in anelectronic IC, consistent with an exemplary embodiment. The componentsof the block diagram of FIG. 6C are similar to those of FIG. 6B and aretherefore not repeated for brevity. The example of FIG. 6C provides anoptical domain FFE implementation that integrates the photonic IC intothe FFE path. In some embodiments, the transmitter sub-system includes aBiCMOS driver IC integrated with a segmented electrode (SE) Mach-Zehndermodulator (MZM) 638. The MZM 638 may leverage a feed-forwardequalization (FFE) scheme, as depicted in FIG. 6C, to extend thebandwidth beyond the RC limit without utilizing electronic equalizationcircuits. By virtue of avoiding equalization, sometimes referred toherein as recombination of the taps in the electrical domain, power andarea are conserved. Accordingly, in the schematic of FIG. 6C, therecombination stage is in the optical domain. Notably, applicants haveobserved error-free 56-Gb/s operation, thereby enabling low latencyFEC-free links for DC connectivity.

FIG. 7 illustrates a schematic of an assembled transmitter sub-system,consistent with an exemplary embodiment. A driver IC 702 is coupled(e.g., via wire bonds) to a modulator IC 704.

Example Implementation and Demonstration of Optical-Domain FFE

As discussed previously, to improve the RC-limited bandwidth of siliconphotonic lumped-element (LE) MZMs with a reverse-biased PN-junctiondiode, traveling-wave (TW) or segmented electrode (SE) driving schemesmay be used. The SE approach discretizes the phase modulator intomultiple shorter segments, reducing the RC of each segment and allowingeach segment to be driven faster, still as a lumped element, at the costof added driver power consumption and chip real estate. However,compared to the more complicated TW approach, the SE scheme circumventsRF transmission line losses, and it does not require impedanceterminations. In a typical non return to zero (NRZ) SE-MZM, each segmentis driven with an identical waveform, delayed slightly to compensate forthe optical delay between segments.

Together with the SE-MZM, applicants have used an FFE scheme to furtherextend the bandwidth without the use of equalized drivers. Referringback to FIG. 6A a generalized FFE block diagram is illustrated, whereina signal is split into multiple taps, and each tap is delayed andweighted with respect to the others before the signals are recombined.Advantageously, such FFE may be used to extend optical device bandwidth.In FIG. 6B, the equalized waveform is entirely generated in a driver ICand subsequently applied to a bandwidth-limited optical device. Putdifferently, the recombination of the taps is performed in theelectrical domain.

In an embodiment where linear elements are used (e.g., analog drivers,phase modulators, and a MZM near quadrature), the order of operationsmay be interchanged, as illustrated in FIG. 6c . Here, the FFE tapsremain separate as they exit the driver IC 636 and are each used todrive isolated phase modulators within a SE-MZM 638. In this way, thetaps (i.e., tap 0 to tap n) are recombined in the optical domain asoptical waveforms rather than voltage or current waveforms. Byindependently adjusting the delays and weights of each tap in FIG. 6C,an optical-domain FFE filter can be realized using the SE-MZM 638without the use of electronic equalization circuits.

Experimental Measurements and Results

By way of demonstrative example and not by limitation, applicants haveconfigured a wire-bond assembly of a custom 6-channel driver IC and a6-segment MZM on a printed circuit board. The driver was fabricated inan IBM (now GlobalFoundries) 130-nm BiCMOS process. Each channelcomprises a 10042 differential input termination, a Cherry-Hooperlimiting amplifier (LA), and a current-mode logic-based output stage(OS) configured to provide a swing of up to 1.6 V across unterminateddifferential outputs that drive the device anodes in a push-pullfashion. The common device cathodes are tied to a voltage supply(V_MOD). Each supply (VCC_LA, VCC_OS, and V_MOD) is powered from anindependent 3.6 V source.

The SE-MZM was fabricated in an IBM (now GlobalFoundries) PhotonicsEnabled 90-nm CMOS process. It comprises six segments of 300-μm lengthelectrically isolated and distributed on a 500-μm pitch matching withthe driver. Each segment has a partially etched silicon waveguide (e.g.,ridge structure), and implants define lateral PN-junction diodesoperated in reverse bias. To tune the MZM bias point, each arm includesa resistive heater of 50-μm length that comprises a thin silicide layerembedded within a ridge waveguide. The measured capacitance of eachsegment with pad is 130 fF, and the measured Vπ-L is 2.04 V-cm. Themodulator has a passive insertion loss of about 2 dB (or 5 dB duringoperation when biased at quadrature). A schematic of the assembly drivenin push-pull configuration is illustrated in FIG. 7 by way of a blockdiagram.

FIG. 8 illustrates an example experimental setup 800 for an FFE testconfiguration. The setup 800 includes a 6-channel pattern generator 802,which supplies the input data streams through 6 passive RF phaseshifters (labeled ‘φ’) 828. In one embodiment, a pseudorandom binarysequence (PRBS) is used. For example, a PRBS is a binary sequence that,while generated with a deterministic algorithm, cannot be readilypredicted. It exhibits statistical behavior similar to a truly-randomsequence.

Light from a 1310-nm butterfly-packaged distributed-feedback (DFB) laser826, after first passing through a polarization controller (PC) 825, isedge-coupled to the assembly using tapered-lensed fibers. Couplinglosses are about 3 dB per facet. The light is then passed through apraseodymium-doped fiber amplifier (PDFA) 822, a tunable opticalbandpass filter (labeled ‘λ’) 820, and variable optical attenuator (VOA)818. A mechanical switch 816 selects between an average optical powermeter 814, a commercial 50-GHz photodetector 812, or a commercial43-Gb/s reference receiver (RX) 810. Eye diagrams are obtained using a60-GHz sampling oscilloscope 808, and bit error rate (BER) measurementsare taken using a BER tester 806. The assembly is mounted on athermo-electric cooler (TEC) and held steady at 36° C.

The assembly 824 is driven using 3 equalization taps: one main tapcomprising 4 SE-MZM segments and two post-cursor taps, which use 1segment each. The post-cursor taps are inverted and delayed with respectto the main tap. Since each driver channel shares common supply and biassettings, the main tap weight is approximately four times that of eachof the post-cursor taps.

FIG. 9A illustrates eye diagrams taken at different transmitter outputspeeds. For example, eye diagrams 902, 904 and 906 are at speeds of 40Gb/s, 50 Gb/s, and 56 Gb/s, respectively. The equalized eyes show nointer-symbol interference up to 40 Gb/s, with slight closure at 50 Gb/s.One drawback with any FFE approach is the reduction in extinction ratio,which in this case is measured to be 1.8 dB. Increasing the number ofsegments further improves the extinction ratio.

An extended BER measurement was performed at 56 Gb/s for more than onehour during which >200 Tb were transmitted and one error was receivedresulting in a BER <5×10¹⁵. In this regard, FIGS. 9B and 9C illustrateBER plots of sensitivity and timing margin, respectively. Thesemeasurements were recorded at the same three data rates. The 40-Gb/ssensitivity in optical modulation amplitude (OMA) is −9.5 dBm withpenalties of 3.5 dB and 9.5 dB in moving to 50 Gb/s and 56 Gb/s,respectively. Timing margins of 0.52 unit intervals (UI), 0.32 UI, and0.07 UI are obtained at 40 Gb/s, 50 Gb/s, and 56 Gb/s, respectively. Alltiming margin curves were recorded by setting the OMA at the receiver tobe 3 dB above the sensitivity point for the specific data rate. Thesensitivity and timing margin degradation incurred when increasing thedata rate are due in part to both the transmitter and the receiver,which is specified only to 43 Gb/s. The measurements reveal no evidenceof a hard error floor. That is, error rates are limited only byamplitude and timing margins and may be improved when taken using ahigher-bandwidth receiver.

The power dissipated by the complete assembly was 2.3 W with 1.4 Wdissipated in the LA stages. The laser consumed a wall-plug power of 0.2W, resulting in an overall transmitter power 2.5 W or about 45 pJ/b at56 Gb/s. In one scenario, the IC discussed herein maintained six fullyindependent channels. Nevertheless, a shared input and pre-amplifier(PA) stage with on-chip tunable delays, as shown in FIG. 6C may have adramatic power saving effect. By simple estimation, reducing the LA fromsix lanes to one could bring the overall efficiency from 45 to 23 pJ/b.

CONCLUSION

While the foregoing has described what are considered to be the bestmode and/or other examples, it is understood that various modificationsmay be made therein and that the subject matter disclosed herein may beimplemented in various forms and examples, and that the teachings may beapplied in numerous applications, only some of which have been describedherein. It is intended by the following claims to claim any and allapplications, modifications, and variations that fall within the truescope of the present teachings.

The components, steps, features, objects, benefits, and advantages thathave been discussed are merely illustrative. None of them, nor thediscussions relating to them, are intended to limit the scope ofprotection in any way. Numerous other embodiments are also contemplated.These include embodiments that have fewer, additional, and/or differentcomponents, steps, features, objects, benefits, and/or advantages. Thesealso include embodiments in which the components and/or steps arearranged and/or ordered differently.

For example, the above embodiments and principles may be applicable withdifferent types of optical modulation device, different delay controlmeans, different gain or loss inducing circuitry, and different signalinverting mechanisms. For example, although the depicted embodimentsintroduce tunable or variable delay devices, fixed delay and tap weightgenerating devices may be incorporated into a circuit implementing theseembodiments. Further, any signal discussed herein may be scaled,buffered, scaled and buffered, converted to another mode (e.g., voltage,current, charge, time, etc.,), or converted to another state (e.g., fromHIGH to LOW and LOW to HIGH) without materially changing the underlyingcontrol method.

Unless otherwise stated, any measurements, values, ratings, positions,magnitudes, sizes, and other specifications that are set forth in thisspecification, including in the claims that follow, are approximate, notexact. They are intended to have a reasonable range that is consistentwith the functions to which they relate and with what is customary inthe art to which they pertain.

Except as stated immediately above, nothing that has been stated orillustrated is intended or should be interpreted to cause a dedicationof any component, step, feature, object, benefit, advantage, orequivalent to the public, regardless of whether it is or is not recitedin the claims.

It will be understood that the terms and expressions used herein havethe ordinary meaning as is accorded to such terms and expressions withrespect to their corresponding respective areas of inquiry and studyexcept where specific meanings have otherwise been set forth herein.Relational terms such as first and second and the like may be usedsolely to distinguish one entity or action from another withoutnecessarily requiring or implying any actual such relationship or orderbetween such entities or actions. The terms “comprises,” “comprising,”or any other variation thereof, are intended to cover a non-exclusiveinclusion, such that a process, method, article, or apparatus thatcomprises a list of elements does not include only those elements butmay include other elements not expressly listed or inherent to suchprocess, method, article, or apparatus. An element proceeded by “a” or“an” does not, without further constraints, preclude the existence ofadditional identical elements in the process, method, article, orapparatus that comprises the element.

What is claimed is:
 1. An optical communication system, comprising: anoptical modulator device including a first and a second waveguidesegment, and configured to modulate an incident optical signal; a firstfeed-forward equalization (FFE) circuit including an inner first tap andan inner second tap, and configured to equalize the first waveguidesegment of the optical modulator; a second FFE circuit including a firstinner tap and a second inner tap, and configured to equalize the secondwaveguide segment of the optical modulator, wherein: an FFErecombination of the first inner tap and the second inner tap of thefirst FFE circuit that creates a first modulation signal to equalize thefirst waveguide segment, is in the electrical domain, an FFErecombination of the first inner tap and the second inner tap of thesecond FFE circuit that creates a second modulation signal to equalizethe second waveguide segment, is in the electrical domain, and an FFErecombination of the first and second modulation signals, operative toequalize a combination of the first and second waveguide segments, is inthe optical domain.
 2. The optical communication system of claim 1,wherein the first modulation signal creates a first outer tap path andthe second modulation signal creates a second outer tap path.
 3. Theoptical communication system of claim 1, wherein for each of the firstand second FFE circuits: the first inner tap is configured to generate afirst inner modulation signal, the second inner tap is configured togenerate a second inner modulation signal, the first inner modulationsignal is attenuated relative to the second inner modulation signal, andthe first and the second inner modulation signals are time-delayed withrespect to each other.
 4. The optical communication system of claim 3,wherein, for each of the first and second FFE circuits, the first innertap comprises a buffer device.
 5. The optical communication system ofclaim 4, wherein, for each of the first and second FFE circuits, thesecond inner tap comprises a tunable buffer device and an inverterdevice coupled to an output of the tunable buffer device.
 6. The opticalcommunication system of claim 4, wherein the first and second FFEcircuits are in BiCMOS technology.
 7. The optical communication systemof claim 1, wherein the first and second inner modulation signals of atleast one of (i) the first tap and (ii) the second tap are inverted withrespect to each other.
 8. The optical communication system of claim 1,wherein the optical modulator comprises a Mach Zehnder Modulator (MZM)having a first arm and a second arm, the first arm including the firstand the second waveguide segment.
 9. The optical communication system ofclaim 1, wherein the optical modulator comprises an Electro-AbsorptionModulator (EAM) having an electro-absorption region including the firstand the second waveguide segment.
 10. The optical communication systemof claim 1, wherein the optical modulator comprises additional waveguidesegments, each waveguide segment being driven by a corresponding innerFFE circuit.
 11. The optical communication system of claim 10, whereineach of the first and second modulation signals generates a change in arefractive index in its corresponding segment that it is driving. 12.The optical communication system of claim 1, wherein, for each of thefirst and second FFE circuits, a delay and amplitude relations betweenthe first inner tap and the second inner tap generates an equalizationsignal, which is encoded onto a phase or an amplitude of the incidentoptical signal passing through the optical modulator.
 13. The opticalcommunication system of claim 1, wherein, each of the first and secondmodulation signals is complementary and operative to control the opticalmodulator device in a push-pull configuration.
 14. The opticalcommunication system of claim 1, wherein the first and second modulationsignals are time-delayed with respect to each other to compensate for anoptical delay between the first and second waveguide segments.
 15. Amethod of optical equalization using two layers of feed-forwardequalization (FFE) on an optical modulator having a first and a secondwaveguide segment, the method comprising: in the first FFE layer, foreach waveguide segment: splitting an electrical modulation signal into afirst inner tap signal and a second inner tap signal; delaying thesecond inner tap signal relative to the first inner tap signal;attenuating an amplitude of the second inner tap signal relative to thefirst inner tap signal; recombining the delayed and attenuated secondinner tap signal with the first inner tap signal in an electrical domainto create an FFE modulation signal; and applying the FFE modulationsignal to the corresponding waveguide segment; and in the second FFElayer: equalizing a combination of the first and second waveguidesegments by recombining each FFE modulation signal in an optical domain.16. The method of claim 15, further comprising, in the first FFE layer,inverting the second inner tap signal upon delaying the second inner tapsignal relative to the first inner tap signal.
 17. The method of claim15, wherein the optical modulator comprises a Mach Zehnder Modulator(MZM) having a first arm and a second arm, the first arm including thefirst and the second waveguide segments.
 18. The method of claim 15,wherein the optical modulator comprises an Electro-Absorption Modulator(EAM) having an electro-absorption region including the first and thesecond waveguide segments.
 19. The method of claim 15, wherein each FFEmodulation signal generates a change in a refractive index in itscorresponding waveguide segment.
 20. The method of claim 15, wherein therecombined FFE optical equalization signal, is encoded onto a phase oramplitude of an incident optical signal passing through the opticalmodulator based on a delay and amplitude relations between the first andsecond modulation signals.
 21. The method of claim 15, wherein the FFEmodulation signals that are applied to the corresponding waveguidesegments are time-delayed with respect to each other to compensate foran optical delay between the corresponding waveguide segments.